Method and apparatus for channel sensitive scheduling in a communication system

ABSTRACT

Method and apparatus for a channel sensitive scheduler for scheduling transmissions in a communication system. The scheduler is defined by a priority function of the channel condition as determined by amount of transmission power needed by a mobile station. In one embodiment the channel condition is determined based on the transmission pilot power of each mobile station and is used to calculate a priority value for each mobile station. The mobile stations are then scheduled to transmit based on the priority value.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a continuation of patentapplication Ser. No. 11/040,871 entitled “METHOD AND APPARATUS FORCHANNEL SENSITIVE SCHEDULING IN A COMMUNICATION SYSTEM” filed Jan. 20,2005, pending, which claims priority to U.S. Provisional Application No.60/538,983, filed Jan. 23, 2004, assigned to the assignee hereof andhereby expressly incorporated by reference herein.

BACKGROUND

1. Field

The present invention pertains generally to communications, and morespecifically to a method and apparatus for channel sensitive schedulingof transmissions in a communication system.

2. Background

Communication systems, and wireless systems in particular, are designedwith the objective of efficient allocation of resources among a varietyof users. Wireless system designers in particular aim to providesufficient resources to satisfy the communication needs of itssubscribers while minimizing costs. Various scheduling algorithms havebeen developed, each based on a predetermined system criteria.

In a wireless communication system employing a Code Division-MultipleAccess (CDMA) scheme or Wideband Code Division Multiple Access (WCDMA)one scheduling method assigns each of the subscriber units code channelsat designated time intervals on a time multiplexed basis. A centralcommunication node, such as a Base Station (BS) or Node B, implementsthe unique carrier frequency or channel code associated with thesubscriber to enable exclusive communication with the subscriber. TDMAschemes may also be implemented in landline systems using physicalcontact relay switching or packet switching. A CDMA system may bedesigned to support one or more standards such as: (1) the“TIA/EIA/IS-95-B Mobile Station-Base Station Compatibility Standard forDual-Mode Wideband Spread Spectrum Cellular System” referred to hereinas the IS-95 standard; (2) the standard offered by a consortium named“3rd Generation Partnership Project” referred to herein as 3GPP; andembodied in a set of documents including Document Nos. 3G TS 25.211, 3GTS 25.212, 3G TS 25.213, and 3G TS 25.214, 3G TS 25.302, referred toherein as the W-CDMA standard; (3) the standard offered by a consortiumnamed “3rd Generation Partnership Project 2” referred to herein as3GPP2, and TR-45.5 referred to herein as the cdma2000 standard, formerlycalled IS-2000 MC, or (4) some other wireless standard. A WCDMA systemmay be designed to support one of more of the same standards listedabove for a CDMA system.

WCDMA is an interference-limited system, which means that neighboringcells and other users limit the uplink and downlink capacity of anysingle cell. To maximize capacity, interference (other signal power)should be minimized. This includes minimizing signal-to-interference(E_(b)/N_(o)) requirements, minimizing overhead channel power, andminimizing control-only channel power. In addition, good phoneperformance includes long battery life. To achieve this goal, the phoneshould minimize its power during dedicated channel transmission andmonitoring of overhead channels.

Accordingly, there is a need for a method and apparatus for channelsensitive scheduling of transmissions in a communication system withapplication to multiple classes of users.

SUMMARY

Embodiments disclosed herein address the above stated needs by providinga means for channel sensitive scheduling of data transmissions in awireless communication system. One embodiment provides a method ofscheduling transmissions in a wireless communication system, comprising:receiving a channel condition indicator sent by a mobile station at ascheduler, determining a priority value for the mobile station using afunction:

Priority(i)=Pilot_Power_Max−Pilot_Power(i),

where Priority(i) is the priority value for the ith mobile user,Pilot_Power_Max is the mobile station's maximum pilot power, andPilot_Power(i) is the mobile station's pilot power at time ofscheduling.

Another embodiment provides for calculating priority values for aplurality of mobile stations as a function of the channel conditionindicator; and selecting at least one of the plurality of mobilestations for a subsequent transmission based on the priority value.Additional embodiments may be based upon the mobile station's transmitpilot power and requested data rate. The mobile station's transmit powermay be determined based upon power control commands in an additionalembodiment.

Further embodiments provide different functions for computing priorityvalues. One further embodiment provides a method of scheduling in awireless communication system, comprising: receiving a channel conditionindicator sent by a mobile station at a scheduler, determining apriority value for the mobile user using a function:

Priority(i)=a(i)*(Pilot_Power_Average(i)/Pilot_Power(i))

where Priority(i) is a priority value for an i-th mobile user,Pilot_Power_Average(i) is a mobile station's pilot power averaged over acertain period of time, Pilot_Power(i) is a mobile station's pilot powerat the moment of scheduling, and a(i) is a weighting factor. Stillfurther embodiments provide for the weighing factor to be based upon themobile station's speed. Yet another embodiment provides for computationof the weighing factor according to the function:

a(i)=(sector_throughput/user_throughput(i))̂b, where 0≦b≦1.

In another embodiment, a computer-readable medium includingcomputer-executable instructions for scheduling transmissions,comprising: processing channel condition indicators received from aplurality of mobile stations; calculating a priority value for each of aplurality of mobile stations; determining a transmission schedule forthe plurality of mobile stations as a function of the priority value.Another embodiment provides a function for calculating the priorityvalue:

Priority(i)=Pilot_Power_Max−Pilot_Power(i),

where Priority(i) is the priority value for the ith mobile station,Pilot_Power_Max is the mobile station's maximum pilot power, andPilot_Power(i) is the mobile user's pilot power at time of scheduling.

Still another embodiment provides a computer program wherein calculatinga priority value uses the function:

Priority(i)=a(i)*(Pilot_Power_Average(i)/Pilot_Power(i))

where Priority(i) is a priority value for an i-th mobile station,Pilot_Power_Average(i) is the mobile station's pilot power averaged overa certain period of time, Pilot_Power(i) is the mobile station's pilotpower at the moment of scheduling, and a(i) is the weighting factor.

A further embodiment provides for calculating the weighting factor iscomputed according to a function:

a(i)=(sector_throughput/user_throughput(i))̂b, where 0≦b ≦1.

Still another embodiment provides a network, comprising: receiving meansfor receiving channel condition indicators from a plurality of mobileusers; means for determining a priority value for each mobile station;means for determining a transmission schedule for a plurality of mobileusers, based on the priority value.

An additional embodiment provides for a network wherein the computationof the priority value is a function of:

Priority(i)=Pilot_Power_Max−Pilot_Power(i),

where Priority(i) is the priority value for the ith mobile station,Pilot_Power_Max is the mobile station's maximum pilot power, andPilot_Power(i) is the mobile station's pilot power at time ofscheduling.

An additional embodiment provides a network wherein the computation ofthe priority value is a function of:

Priority(i)=a(i)*(Pilot_Power_Average(i)/Pilot_Power(i))

where Priority(i) is a priority value for an i-th mobile station,Pilot_Power_Average(i) is the mobile station's pilot power averaged overa certain period of time, Pilot_Power(i) is the mobile station's pilotpower at the moment of scheduling, and a(i) is the weighting factor.

Yet another embodiment provides for a network, wherein the weightingfactor is computed according to the function:

a(i)=(sector_throughput/user_throughput(i))̂b, where 0≦b≦1.

An additional embodiment provides an apparatus in a wirelesscommunication system, comprising:a processing element; and a memorystorage element coupled to the processing element, the memory storageelement adapted for storing computer-readable instructions forimplementing: means for receiving a channel condition indicator from aplurality of mobile stations; means for computing a priority value foreach mobile station based on the channel condition indicator; and meansfor scheduling the plurality of mobile stations based on the computedpriority values.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the presently disclosed methodand apparatus will become more apparent from the detailed descriptionset forth below when taken in conjunction with the drawings in whichlike reference characters identify correspondingly throughout andwherein:

FIG. 1 is a wireless communication system according to an embodiment ofthe invention.

FIG. 2 is a wireless communication system supporting a channel sensitivescheduling algorithm.

FIG. 3 illustrates the interaction of outer and inner loop power controlin a wireless communication system.

FIG. 4 illustrates power control for a User Equipment (UE) during softhandover.

FIG. 5 illustrates uplink scheduling

FIG. 6 is a flow diagram of a channel sensitive scheduler using greedyfilling according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A modern day communication system is desired to support a variety ofapplications. One such communication system is a code division multipleaccess (CDMA) system which conforms to the “TIA/EIA-95 MobileStation-Base Station Compatibility Standard for Dual-Mode WidebandSpread Spectrum Cellular System” and its progeny, hereinafter referredto as IS-95. The CDMA system allows for voice and data communicationsbetween users over a terrestrial link. Another communication system is awideband code division multiple access (WCDMA) system. The use of CDMAtechniques in a multiple access communication system is disclosed inU.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESSCOMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S.Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMSIN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee ofthe present invention and incorporated by reference herein.

In a CDMA system or WCDMA system, communications between users areconducted through one or more base stations. In wireless communicationsystems, forward link refers to the channel through which signals travelfrom a base station to a subscriber station, and reverse link refers tothe channel through which signals travel from a subscriber station to abase station. By transmitting data on a reverse link to a base station,a first user on one subscriber station communicates with a second useron a second subscriber station. The base station receives the data fromthe first subscriber station and routes the data to a base stationserving the second subscriber station. Depending on the location of thesubscriber stations, both may be served by a single base station ormultiple base stations. In any case, the base station serving the secondsubscriber station sends the data on the forward link. Instead ofcommunicating with a second subscriber station, a subscriber station mayalso communicate with a terrestrial Internet through a connection with aserving base station. In wireless communications such as thoseconforming to IS-95, forward link and reverse link signals aretransmitted within disjoint frequency bands.

WCDMA systems use slightly different terminology than CDMA systems.There are three major subsystems in a WCDMA system. User Equipment (UE)may be a mobile, a fixed station, a data terminal or other device. A UEincludes a Universal Subscriber Identity Module (USIM) which contains auser's subscription information. The Access Network (AN) includes theradio equipment for accessing the network. It may be either UniversalTerrestrial Radio Access Network (UTRAN) or Global System for Mobilecommunications/Enhanced Data rates for GSM Evolution (GSM/EDGE) RadioAccess Network (GSM/EDGE RAN). The Core Network (CN) includes theswitching and routing capability for connecting to either the PublicSwitched Telephone Network (PSTN) for circuit switched calls or to aPacket Data Network (PDN) for packet switched calls. The Core Networkalso includes mobility and subscriber location management andauthentication services.

FIG. 1 serves as an example of a communications system 100 that supportsa number of users and is capable of implementing at least some aspectsand embodiments presented herein. Any of a variety of algorithms andmethods may be used to schedule transmissions in system 100. System 100provides communication for a number of cells 102A through 102G, each ofwhich is serviced by a corresponding base station 104A through 104G,respectively. In the exemplary embodiment, some of base stations 104have multiple receive antennas and others have only one receive antenna.Similarly, some of base stations 104 have multiple transmit antennas,and others have single transmit antennas. There are no restrictions onthe combinations of transmit antennas and receive antennas. Therefore,it is possible for a base station 104 to have multiple transmit antennasand a single receive antenna, or to have multiple receive antennas and asingle transmit antenna, or to have both single or multiple transmit andreceive antennas.

Terminals 106 in the coverage area may be fixed (i.e., stationary) ormobile. As shown in FIG. 1, various terminals 106 are dispersedthroughout the system. Each terminal 106 communicates with at least oneand possibly more base stations 104 on the downlink and uplink at anygiven moment depending on, for example, whether soft handoff is employedor whether the terminal is designed and operated to (concurrently orsequentially) receive multiple transmissions from multiple basestations. Soft handoff in CDMA communications systems is well known inthe art and is described in detail in U.S. Pat. No. 5,101,501, entitled“Method and system for providing a Soft Handoff in a CDMA CellularTelephone System”, which is assigned to the assignee of the presentinvention.

The downlink refers to transmission from the base station to theterminal, and the uplink refers to transmission from the terminal to thebase station. In the exemplary embodiment, some of terminals 106 havemultiple receive antennas and others have only one receive antenna. InFIG. 1, base station 104A transmits data to terminals 106A and 106J onthe downlink, base station 104B transmits data to terminals 106B and106J, base station 104C transmits data to terminal 106C, and so on.

Increasing demand for wireless data transmission and the expansion ofservices available via wireless communication technology have led to thedevelopment of specific data services. As the amount of data transmittedand the number of transmissions increases, the limited bandwidthavailable for radio transmissions becomes a critical resource.Additionally, interference becomes a significant problem. Channelconditions may affect which transmissions may be sent efficiently. Thereis a need, therefore, for a channel sensitive means of schedulingtransmissions in a wireless communication system. In the exemplaryembodiment, system 100 illustrated in FIG. 1 is consistent with a WCDMAtype system having High Data Rate (HDR) service.

A WCDMA system manages data transmission using the Medium Access Control(MAC) layer of the system architecture. Data transmission utilizes theselection of a Transport Format Combination (TFC). TFC selection isperformed by the MAC layer. For each radio frame, the Physical Layerrequests data from the MAC layer. The MAC queries the Radio Link Control(RLC) to determine how much data is available to send in order todetermine how much data the MAC layer can deliver to the Physical Layerfor transmission. The Transport Format Combination Indicator (TFCI)represents the TFC in use. As an example, consider a packet switcheddata call. The Physical Layer channel is configured to carry variablelength frames up to a maximum data rate. Based on available data on theRLC logical channels, MAC selects a transport format combination thatultimately determines the data rate of the physical channel on a frameby frame basis.

Signaling data is intermittent, so often there will be no Protocol DataUnits (PDU) available to send on the Signal Radio Bearers (SRB).Alternatively, there may be data available for transport on multipleSRBs at the same time. In the latter case, the MAC uses logical channelpriorities to determine which SRB will send the data.

Packet switched data is inherently bursty, so the amount of dataavailable to send may vary from frame to frame. When more data isavailable, MAC may choose a higher data rate. When both signaling anduser data are available, MAC should choose between them to maximize theamount of data sent from the higher priority channel.

A Transport Block is the basic unit of data exchanged between the MACand the Physical Layer. A Transport Block is a set of zero or moretransport blocks. For a given transport channel, the physical layerrequests data from the MAC every Transmission Time Interval (TTI). Theadvantage of breaking a large block of data into a set of smaller blocksis that each of the smaller blocks can have a separate Cyclic RedundancyCheck (CRC). An error may occur in one block, leaving other blocksunaffected. If there was only one CRC for a large block of data, asingle error could cause the entire block to be discarded.

A Transport Format defines the transport block size and number of blocksthat MAC may deliver to the physical layer during a TTI. The TransportFormat Set defines all of the valid Transport Formats for each transportchannel. For example, to support a 57.6 kbps circuit switch radio accessbearer for streaming data, the transport block size is 576 bits, with upto four blocks that could be sent in one transport block, with a 49 msTTI. The Transport Formats are labeled from TF0 to TF3 for the exampleabove.

Multiple transport channels may be multiplexed onto a Coded CompositeTransport Channel (CCTrCh). Each transport channel has a TransportFormat Set defined for it. A Transport Format Combination (TFC) definesa combination of Transport Formats, one for each transport channel,which can be used simultaneously across the transport channels mapped toa CCTrCh. For example, TFC for each typical voice configuration selectsone block from each of the dedicated channels (DCH) to which the circuitswitched radio access bearer (CS RAB) subflows are mapped and one blockfrom the DCH to which the four SRBs are mapped.

As part of the CCTrCh configuration, MAC is given a Transport FormatCombination Set (TFCS). The TFCS lists all of the allowed TFCs for thatCCTrCh. At each radio frame boundary, MAC is responsible for selecting aTFC from the TFCS. MAC bases this choice on the buffer status of eachlogical channel, the relative priorities of each logical channel, andquality of service parameters for each logical channel. Depending on thenature of each logical channel, MAC may deal in a different manner withdata that could not be sent at a particular TTI boundary. For example,non-realtime data may be queued for future transmission, while data forstreaming video may be discarded.

The Transport Format Combination Indicator (TFCI) is the index into theTFCS for a particular TFC. The physical channel may be configured totransmit the TFCI in each radio frame, allowing the receiver to quicklydetermine the TFC that was used in each radio frame.

Every minimum TTI, MAC performs Transport Format Combination (TFC)selection to determine the number of bits to be transmitted from eachtransport channel. When the transport blocks are delivered to thephysical layer for transmission, MAC indicates which TFC was chosen. MACrepresents the TFC using a Transport Format Combination Indicator(TFCI), which is then transmitted on the dedicated physical controlchannel.

One example of a communication system supporting data transmissions andadapted for scheduling transmissions to multiple users is illustrated inFIG. 2. FIG. 2 illustrates the operation of the base stations 104 fromFIG. 1. FIG. 2 is detailed hereinbelow, wherein specifically, a basestation, or Node B, 220 and base station controller 210 interface with apacket network interface 206. Base station controller 210 includes achannel scheduler 212 for implementing a scheduling algorithm fortransmissions in system 200. The channel scheduler 212 determines TTIduring which data is to be transmitted as described above.

In addition, the channel scheduler 212 selects the particular data queuefor transmission. The associated quantity of data to be transmitted isthen retrieved from a data queue 230 and provided to the channel element226 for transmission to the remote station associated with the dataqueue 230. As discussed below, the channel scheduler 212 selects thequeue for providing the data, which is transmitted in a following TTI.

Note that it may be possible for the user to receive a packet correctlyeven if only a portion of the packet is transmitted. This occurs whenthe channel condition is better than anticipated by the user. In thatcase, the user may send an “ACK” signal to the base station indicatingthat the packet is already correctly received and the remaining portionsof the packet need not be transmitted. When this happens, the entiredata packet is effectively transmitted to the user over a shorterservice interval thereby increasing the effective data rate at which thepacket is transmitted. The base station then reassigns the time slotsthat were originally scheduled to transmit the remaining portions ofthat packet to transmit another packet either to the same user or to adifferent user. This process is generally referred to as AutomaticRepeat reQuest (ARQ).

In a system supporting ARQ, a data packet is scheduled for apredetermined number of transmissions, wherein each transmission mayinclude different information. The multiple transmissions are interposedwith other packets sequentially. When a receiver has received sufficientinformation to decode and process the packet, the receiver sends anindication to the transmitter that no further information is needed forthe current packet. The transmitter is then free to schedule the slotsoriginally scheduled for the current packet to another packet. In thisway, the system resources are conserved and the transmission time to thereceiver is reduced.

A block diagram illustrating the basic subsystems of an exemplaryvariable rate communication system is shown in FIG. 2. Base stationcontroller 210 interfaces with packet network interface 206, PublicSwitched Telephone Network, PSTN, 208, and all base stations or Node Bsin the communication system (only one base station 220 is shown in FIG.2 for simplicity). Base station controller 210 coordinates thecommunication between remote stations in the communication system andother users connected to packet network interface 206 and PSTN 208. PSTN208 interfaces with users through a standard telephone network (notshown in FIG. 2).

Base station controller 210 may contain many selector elements 216,although only one is shown in FIG. 2 for simplicity. Each selectorelement 216 is assigned to control communication between one or morebase stations or Node Bs 220 and one remote station (not shown). Ifselector element 216 has not been assigned to a given remote station,call control processor 218 is informed of the need to page the remotestation. Call control processor 218 then directs base station 220 topage the remote station.

Data source 202 contains a quantity of data, which is to be transmittedto a given remote station. Data source 202 provides the data to packetnetwork interface 206. Packet network interface 206 receives the dataand routes the data to the selector element 216. Selector element 216then transmits the data to each base station 220 in communication withthe target remote station. In the exemplary embodiment, each basestation 220 maintains a data queue 230, which stores the data to betransmitted to the remote station.

The data is transmitted in data packets from data queue 230 to channelelement 226. In the exemplary embodiment, on the forward link, a “datapacket” refers to a quantity of data which is a maximum of 1024 bits anda quantity of data to be transmitted to a destination remote stationwithin a predetermined “time slot” (such as ≈1.667 msec). For each datapacket, channel element 226 inserts the necessary control fields. In theexemplary embodiment, channel element 226 performs a Cyclic RedundancyCheck, CRC, encoding of the data packet and control fields and inserts aset of code tail bits. The data packet, control fields, CRC parity bits,and code tail bits comprise a formatted packet. In the exemplaryembodiment, channel element 226 then encodes the formatted packet andinterleaves (or reorders) the symbols within the encoded packet. In theexemplary embodiment, the interleaved packet is covered with a Walshcode, and spread with the short PNI and PNQ codes. These PNI and PNQcodes are well known in CDMA wireless systems. The spread data isprovided to RF unit 228 which quadrature modulates, filters, andamplifies the signal. The forward link signal is transmitted over theair through an antenna to the forward link and to the mobile station orUE.

At the remote station, the forward link signal is received by an antennaand routed to a receiver. The receiver filters, amplifies, quadraturedemodulates, and quantizes the signal. The digitized signal is providedto a demodulator (DEMOD) where it is despread with the short PNI and PNQcodes and decovered with the Walsh cover. The demodulated data isprovided to a decoder which performs the inverse of the signalprocessing functions done at base station 220, specifically thede-interleaving, decoding, and CRC check functions. The decoded data isprovided to a data sink.

The hardware, as pointed out above, supports variable rate transmissionsof data, messaging, voice, video, and other communications over theforward link. The rate of data transmitted from the data queue 230varies to accommodate changes in signal strength and the noiseenvironment at the remote station, or UE. The UEs send informationconcerning receipt of the data, including ACK/NACK messages to the NodeB. In addition, information on transmission power is also transmitted.Accordingly, circuitry at the remote station measures the signalstrength and estimates the noise environment at the remote station todetermine the rate information for future transmission.

The signal transmitted by each UE travels through a reverse link channeland is received at base station 220 through a receive antenna coupled toRF unit 228. In the exemplary embodiment, the pilot power and data rateinformation is demodulated in channel element 226 and provided to achannel scheduler 212 located in the base station controller 210 or to achannel scheduler 232 located in the base station 220. In a firstexemplary embodiment, the channel scheduler 232 is located in the basestation 220. In an alternate embodiment, the channel scheduler 212 islocated in the base station controller 210, and connects to the selectorelements 216 within the base station controller 210.

In the first-mentioned exemplary embodiment, channel scheduler 232receives information from data queue 230 indicating the amount of dataqueued up for each remote station, also called “queue size”. Channelscheduler 232 then performs scheduling based on channel condition foreach UE serviced by base station 220. If queue size is utilized for ascheduling algorithm used in the alternate embodiment, channel scheduler212 may receive queue size information from selector element 216.

During the transmission of a packet to one or more users, the userstransmit an “ACK” signal after each time slot containing a portion ofthe transmitted packet. The ACK signal transmitted by each user travelsthrough a reverse link channel and is received at base station 220through a receive antenna coupled to RF unit 228. In the exemplaryembodiment, the ACK information is demodulated in channel element 226and provided to a channel scheduler 212 located in the base stationcontroller 210 or to a channel scheduler 232 located in the base station220. In a first exemplary embodiment, the channel scheduler 232 islocated in the base station 220. In an alternate embodiment, the channelscheduler 212 is located in the base station controller 210, andconnects to all selector elements 216 within the base station controller210.

Embodiments of the present invention are applicable to other hardwarearchitectures, which can support variable rate transmissions. Thepresent invention can be readily extended to cover variable ratetransmissions on the reverse link. For example, the base station 220measures the strength of the signal received from the remote stationsand estimates the noise environment and power requirements to determinea rate of receiving data from the remote station. The base station 220then transmits to each associated remote station the rate at which datais to be transmitted in the reverse link from the remote station. Thebase station 220 may then schedule transmissions on the reverse linkbased upon the different data rates on the reverse link in a mannersimilar to that described herein for the forward link.

Also, a base station 220 of the embodiment discussed above transmits toa selected one, or selected ones, of the remote stations to theexclusion of the remaining remote stations associated with the basestation or Node B 220 using a Code Division-Multiple Access, CDMA,scheme or a WCDMA scheme. At any particular time, the base station 220transmits to the selected one, or selected ones, of the remote stationby using a code, which is assigned, to the receiving base station(s) orNode Bs 220. However, this scheme is also applicable to other systemsemploying different Time Division-Multiple Access, TDMA, methods forproviding data to select base station(s) 220, to the exclusion of theother base stations 220, for allocating transmission resourcesoptimally.

The channel scheduler 212 schedules the variable rate transmissions onthe forward link. The channel scheduler 212 receives the queue size,which is indicative of the amount of data to transmit to a remotestation, and messages from remote stations. The channel scheduler 212preferably schedules data transmissions to achieve the system goal ofmaximum data throughput while minimizing interference.

As shown in FIG. 1, remote stations are dispersed throughout thecommunication system and can be in communication with zero or one basestation or Node B on the forward link. In the exemplary embodiment,channel scheduler 212 coordinates the forward link data transmissionsover the entire communication system. A scheduling method and apparatusfor high speed data transmission are described in detail in U.S. Pat.No. 6,335,922 entitled “Method and Apparatus for Forward Link RateScheduling,” issued Jan. 1, 2002, assigned to the assignee of thepresent invention and hereby expressly incorporated by reference.

According to an embodiment, the channel scheduler 212 is implemented ina computer system, which includes a processor, Random Access Memory,RAM, and a program memory for storing instructions to be executed by theprocessor (not shown). The processor, RAM and program memory may bededicated to the functions of the channel scheduler 212. In otherembodiments, the processor, RAM and program memory may be part of ashared computing resource for performing additional functions at thebase station controller 210. In the exemplary embodiment, a generalizedscheduler is applied to the system 200 illustrated in FIG. 2 and isdetailed hereinbelow. Those modules within the BSC 210 and BS 220 usedto implement a channel sensitive scheduling function for scheduling datatransmissions is discussed below.

Given the growing demand for wireless data applications, the demand forvery efficient wireless data communication systems has increasedsignificantly. The IS-95 standard is capable of transmitting trafficdata and voice data over the forward and reverse links. In accordancewith the IS-95 standard, the traffic data or voice data is partitionedinto code channel frames that are 20 milliseconds wide with data ratesas high as 14.4 Kbps. In an IS-95 system, each subscriber station isallocated at least one of a limited number of orthogonal forward linkchannels. While the communication between a base station and asubscriber station is ongoing, the forward link channel remainsallocated to the subscriber station. When data services are provided inan IS-95 system, a forward link channel remains allocated to asubscriber station even during times when there is no forward link datato be sent to the subscriber station.

A significant difference between voice services and data services is thefact that the former typically imposes stringent and fixed delayrequirements. Typically, the overall one-way delay of speech frames arespecified to be less than 100 milliseconds. In contrast, the data delaycan become a variable parameter used to optimize the efficiency of thedata communication system.

Yet another significant difference between voice services and dataservices is that the former typically requires a reliable communicationlink which, in the exemplary CDMA or WCDMA communication system, isprovided by soft handoff. Soft handoff results in redundanttransmissions from two or more base stations to improve reliability.However, this additional reliability is not required for datatransmission because the data packets received in error can beretransmitted. For data services, the transmit power used to supportsoft handoff can be more efficiently used for transmitting additionaldata.

Transmission delay required to transfer a data packet and the averagethroughput rate are two attributes used to define the quality andeffectiveness of a data communication system. Transmission delay usuallydoes not have the same impact in data communication as it does for voicecommunication, but it is an important metric for measuring the qualityof the data communication system. The average throughput rate is ameasure of the efficiency of the data transmission capability of thecommunication system. Throughput rate is also affected by the amount ofpower required for transmission. There is a need for a channel sensitivemethod of scheduling transmissions based on power requirements. Powerrequirements in a wireless communication system as discussed below.

WCDMA is an interference-limited system, which means neighboring cellsand other users limit the uplink and downlink capacity of any singlecell. To maximize capacity, other signal power, which producesinterference, must be minimized. This includes minimizingsignal-to-interference (Eb/No) requirements, minimizing overhead channelpower, and minimizing control-only channel power. Good mobile phoneperformance includes long battery life. To accomplish this, the mobilephone must minimize its power during dedicated channel transmission,monitoring overhead channels, and transmitting using the minimum powersetting for transmission.

A robust CDMA or WCDMA system requires good power control. Power controlminimizes the transmit power of the mobile or UE and the network.Because CDMA and WCDMA systems are interference limited, reducing thepower from all users increases the capacity of the system.Inefficiencies in power control reduce overall system capacity.

The most basic problem in power control is the near-far problem.Close-in transmitters are heard more easily than transmitters furtheraway. Power control causes these transmitters to transmit at such apower level that their received signal is the same or nearly the same asa transmitter located further away.

Efficient power control requires fast feedback to minimize systemcapacity loss. Fast power control is known as inner loop power controland runs at 1500 Hz. Thus, the transmitter gets commands 1500 times asecond from the receiver to increase or decrease power.

For voice calls good quality of service is near a 1% block error rate(BLER). To maintain a 1% BLER, a certain signal-to-interference (SIR)may be required. If the user is in a bad fading environment, such asmoving fast in a cluttered environment, then the user needs a higher SIRtarget than a user in a better fading environment, such as moving slowlyin a clutter-free environment. Because both users require a 1% BLER, thepower control must find the correct SIR target. The process of findingthe correct SIR target is called outer loop power control. Differencesin SIR targets cause differences in receive power.

A closed loop process controls transmission power on both the downlinkand uplink. Closed loop power control is a three step process. Atransmission is made, a measurement is made at the receiver, and feed isprovided to the transmitter indicating whether the power should beincreased or decreased.

The closed loop process can eventually correct the mobile or UE'stransmit power regardless of the initial transmit level. Significantgain can be achieved if the UE's initial transmit level is close to theappropriate power. Selection of a metric is affected by the speedrequired of the closed loop process. Block error rate (BLER) is a goodmetric, however, measuring BLER can be a time consuming process. If afaster response is needed, Eb/No, may be a better selection. For quickresponse to power control commands, multiple commands are sent everyradio frame.

FIG. 3 shows the interaction of the outer loop and inner loop controlmechanisms. An SIR target algorithm based on BLER may be adjustedslowly. Since BLER is based on cyclic redundancy checks (CRC), andadaptive multi-rate (AMR) voice CRCs are received on 20 ms transmissiontime interval (TTI), the fastest that the outer loop power control canbe adjusted is 50 times per second.

Inner loop power control utilizes the SIR estimate. The SIR estimate isusually calculated every slot (15 times per 10 ms radio frame), sincethe dedicated physical control channel pilot power is present in everyshot. The inner loop is given the SIR target. If the SIR estimate isgreater than the SIR target, the inner loop signals for a decrease intransmitter power. If the SIR target is less than the SIR target, theinner loop signals for an increase in transmitter power. This happensquickly, approximately 1500 times per second to rapidly compensate forquickly changing fading conditions.

The inner loop and outer loop interact. The inner loop uses a slowlychanging SIR target. The outer loop delivers the SIR target to the innerloop. See FIG. 3 for a depiction of this interaction.

The UE performs its own downlink closed loop power control algorithm.The UE may measure the BLER over a number of frames and increases anddecreases the SIR target. Based on the SIR target and the SIR estimate,the UE directs the universal terrestrial radio access network (UTRAN) toincrease or decrease the UE's dedicated channel transmit power. Therange of power adjustment for a Node B is typically around 20 db.

The downlink or inner loop power control runs at either 1500 or 500 Hz.The power control command is communicated to the UE and is sent quicklyto respond to changing channel conditions. When there are multiple NodeBs, the UE is sending a single up or down command to multiple Node Bs. Aweaker link may be told to decrease power, which will reduce the overallinterference of the system. If a stronger Node B signal degrades, the UEsignals a power increase command. Upon receipt of the power up command,all Node Bs increase their downlink power.

Uplink power control varies from the downlink power control describedabove. UEs may be located anywhere within the cell. One UE may bethousands of meters away from the cell, while another UE may be only afew hundred meters away. Thus, users experience greatly varying amountsof path loss due to their varying distance from the cell and theirvarying multipath environments. Path loss can exceed 80 db for example.Each UE must be carefully power controlled to ensure that transmissionarrive at the cell at an appropriate level, including initialtransmissions, to minimize interference to other users. For initialpower settings, the UE uses an open loop estimate. For the open loopestimation the UE receives signaled parameters and makes channelmeasurements. During the close loop power control operation the UE isprovided feedback that minimizes its interference.

A UE involved in a soft handover may receive conflicting power controlcommands from the different Node Bs. The UE resolves the conflict byapplying a simple rule: if any Node B commands the UE to reduce power,the UE will reduce power. This is called the “OR of downs”. In the eventof a multi-cell (same Node B) handoff, the UE should receive identicalcommands from the two cells. Knowing this, the UE “soft combines” thebits before making a decision on the value of the bit. Here, there is noOR of the downs because if the signal is from two cells but the sameNode B, the signal likely experiences the same general fadingenvironment. The UE can tell if the two radio links are from the sameNode B based on the TPC index, as discussed previously.

FIG. 4 illustrates a UE in soft handover. A UE 404, is in soft handoverwith Node B1 406 and Node B2 402. The system 400 includes both Node Bsand the UE.

During handover, there can be up to six sets of TPC indices, one indexfrom each Node B. If the TPC index is the same, it means those cellscorrespond to the same Node B. If the Node Bs are different, then theTPC indices will be different. The UE powers down if any of the Node Bstransmit a power down command.

The embodiments described herein are applicable to a variety ofscheduling algorithms and prioritizations, and is not limited to thosedescribed herein. For clarity, several scheduling algorithms will bediscussed to provide examples of a generalized scheduler and variousimplementations.

Embodiments of the present invention are directed to a system andapparatus for scheduling transmissions based on channel sensitivescheduling.

Channel sensitive scheduling depends upon some enhancements to theuplink portion of the WCDMA system. The uplink transmissions can bescheduled by the Node B and physical frames retransmitted and softcombined. The TTI may be 2 ms, which is used for UEs that are not insoft handover. For UEs in soft handover, a TTI of 10 ms may be used.However, the network decides which UE is assigned 10 ms and which UE toassign 2 ms.

Short TTI enable channel sensitive scheduling. Channel sensitivescheduling can significantly increase uplink throughput and reducedelay. Any practical scheduling algorithm should provide at least somefairness, in order to ensure that every UE in the system receives atleast some throughput. UEs are scheduled when their transmit power islow compared to the average transmitted power, thus minimizinginterference to the system, delay, and maximizing throughput.

FIG. 5 shows a UE on the uplink that is in soft handover with Node B1and Node B2. A single Node B is the serving node. Only the serving NodeB schedules the uplink traffic. In the example shown in FIG. 5, Node B1is the serving node and schedules uplink traffic. All Node Bs in softhandover decode physical layer frames and acknowledge successfuldecoding of a physical layer frame. Any needed retransmissions aresynchronous and follow the first transmission at a predetermined timeinterval. Soft combining of the retransmissions is performed at the NodeB. The radio network controller (RNC) is aware of the serving node foreach UE.

The objective of channel sensitive scheduling is to reduce interferenceto other cells and to better utilize the available uplink resources,resulting in higher throughput and lower delay. UEs are scheduled whenthe channel condition is good. When the channel condition is good, thetransmitted pilot power is low and interference toward other cells isless for the same amount of data transmitted. Only the first sub-packetis scheduled. Any retransmissions needed are transmitted at apre-determined time shortly after the initial transmission and are notindependently scheduled. This is because of the nature of the hybrid ARQmethod, which fixes the time for any retransmissions.

The hybrid ARQ method is employed because it is link efficient. Initialtransmissions are not targeted to achieve the targeted frame or blockerror rate. Rather, the frame or block error rate is intended to beachieved after any needed retransmissions have occurred. Theretransmissions in synchronous hybrid ARQ operation are defined inadvance. For example, the maximum number of retransmissions allowed maybe three. The retransmissions are scheduled at specific times in thetransmission queue and those times are defined when the system isconfigured for operation. Therefore, the first retransmission can bescheduled according to channel conditions and scheduling the firstretransmission automatically schedules the remaining retransmissioninstances.

The system operating point does not change with channel sensitivescheduling. A 1% to 5% frame error rate or block error rate remains ineffect. To achieve that quality of service level a user may need mayneed to transmit with more power in poor channel conditions, orconversely, may be able to achieve that quality of service with a lowertransmit power level. While the goal of channel sensitive scheduling isto schedule users with the lowest transmission power levels first, thepower level is related to the user's requested data rate. A higher datarate generally requires more transmit power. For example, a userencountering good channel conditions and a user in bad channelconditions may have identical transmit power level requests. The userwith better channel conditions would use a higher data rate fortransmission, while the user in bad channel conditions would use a lowerdata rate. For improved throughput, the user with the higher data ratewould then be scheduled ahead of the user with the lower data rate.However, if both users request the same data rate, then the user withbetter channel conditions would use less transmit power and would bescheduled ahead of the user in bad channel conditions who requires moretransmit power to achieve the same data rate.

FIG. 6 is a flow diagram explaining the method of the invention. Themethod, 600, begins with the start block, 602, with transmissions toschedule. The scheduler is located in the Node B and maintains a list ofall UEs that are in soft handover with the Node B. The scheduler assignstransmission resources only to the UEs for which the Node B has the bestdownlink conditions.

Scheduling is initiated when the Node B updates queue information foreach UE it schedules, step 604 in FIG. 6. The queue consists of the datathat the UEs requesting to transmit for all the UEs scheduled by theNode B.

The scheduler computes the maximum TFC allowed in the TFCS for each UEto be scheduled in step 606. Computing the maximum TFC consists of theprocess described above and simplifies the computation of the maximumdata rate.

At step 608 the scheduler updates the available resources. This involvesthe allowable rise over thermal for the wireless system and thepreselected system operating point. For example, 4 dB may be theallowable rise over thermal for the system. The rise over thermal isbased on the received power of each UE and includes an estimate of theinterference seen by each UE. Also included is the contribution ofautonomous transmissions and transmissions of the non-scheduled UEs thatare in soft handover at the moment of estimation.

After completing the update estimate, the scheduler in step 610 updatesthe statistics on the average pilot transmission power of each UE on thescheduling list.

In step 612 the scheduler updates the information on the UE pilottransmission power, when the feedback is available. Once the update iscompleted the scheduler creates a priority list based on computations ofthe scheduling algorithm in step 614.

The scheduling algorithm has two major characteristics: prioritizationof UE requests and greedy filling for maximum capacity utilization. TheUE requests are prioritized according to the results of the priorityfunction calculation. Each UE has a priority count associated with it.Initially the priority of a UE is set to zero. When a new UE enters thesystem which the Node B is serving or its buffer becomes non-empty afterbeing idle due to the lack of data, its priority is set to

min{PRIORITY_(i), ∀i such that UE_(i) has cell j as the primary cell}

At the moment of scheduling, the scheduler, located at the Node B, isaware of the pilot power level of all users it schedules. The schedulercreates a priority list by sorting the priority values, computedaccording to the following two alternative algorithms.

Compute Max Threshold according to:

Priority(i)=Pilot_Power_Max−Pilot_Power(i)

where Priority(i) is the priority value for the i-th user,Pilot_Power_Max is the UE's maximum pilot power, and Pilot_Power(i) isthe user's pilot power at the moment of scheduling.Compute Average Threshold according to:

Priority(i)=a(i)*(Pilot_Power_Average(i)/Pilot_Power(i))

where Priority(i) is the priority value for the i-th user,Pilot_Power_Average(i) is the user's pilot power averaged over a certainperiod of time, Pilot_Power(i) is the user's pilot power at the momentof scheduling, and is the weighting factor. The a(i) is chosen such thatit reflects user's speed. Another alternative selection for a(i) isallow a(i) to reflect the user's throughput, so that the user receivessome capacity and is not ignored in scheduling. Another alternative isthat a(i) reflect user's throughput, so that the user is not starved.For example: a(i)=(sector_throughput/user_throughput(i))̂b, where 0≦b≦1;

-   -   a(i) takes a larger value for low speed users: most of the gain        of channel sensitive scheduling is seen with low speed users        since channel can be tracked and channel conditions do not        change rapidly, allowing the scheduler to take advantage of the        channel. Low speed users are prioritized over high speed users        in order to better utilize channel conditions, increase        throughput, and decrease delay.

Once the priority list in step 614, has been created the schedulerperforms greedy filling in step 616. “Greedy filling” is a technique formaximum capacity of a channel. At this point the scheduler has createdthe priority list and the order of transmission for the UEs is known.The scheduler knows the amount of resources available, which istypically in the form of amount of rise over thermal. The schedulertakes the first UE on the priority list and notes the data raterequested. The scheduler assumes that the UE will take the maximum datarate available and then calculates the resulting rise over thermal forthe requested data rate. If the amount of data to be transmitted doesnot require all of the available capacity, the scheduler then examinesthe next UE and determines if the remaining capacity can accommodate thesecond UE. This process continues as long as there are UEs to bescheduled and remaining capacity. If a UE cannot be completely fit intothe remaining available capacity, then the data rate granted that UE islowered until the capacity is filled. Thus, the last mobile scheduledmay be assigned a lower data rate than requested.

Once the scheduler has completed the scheduling in step 616, the data istransmitted in step 618. Transmission occurs in the order determined bythe scheduler in the Node B.

A variety of possible implementations of channel sensitive schedulingare possible. One embodiment provides for the user transmit power to beestimated at the scheduler using the power control commands sent on thedownlink. As pointed out in FIG. 5, it is assumed that the schedulingcell is the serving cell. This assumption may be impaired due to powercontrol command errors and the fact that a user in soft handover obeyspower control commands from a non-serving Node B.

To combat this situation, occasional synchronization of the actualtransmit power and the estimated transmit power is needed. This may bedone by sending 4 bits containing the transmit power information sentevery 20 ms.

Additionally, users in soft handover may need to send a feedback messageto the serving cell that sent the power control command in order toavoid the drift of transmit power estimation that occurs when thenon-serving cell power command is applied.

UEs can keep track of the average transmit power used and can beperiodically configured by the serving Node B to send an indicator thatinforms the scheduler whether the current transmit power is above orbelow the average transmit power. This creates low overhead, since only1 bit may be needed. This method may be used in conjunction with thetransmit power estimation based on power control commands. Anydiscrepancies between the relative position of the estimated transmitpower to the threshold and the reported position of the UE may be usedto pinpoint the problem and invoke resynchronization of the actualtransmit power and the estimated transmit power.

Thus, a novel and improved method and apparatus for schedulingtransmissions in a communications system has been described. Those ofskill in the art would understand that the data, instructions, commands,information, signals, bits, symbols, and chips that may be referencedthroughout the above description are advantageously represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof. Those of skillwould further appreciate that the various illustrative logical blocks,modules, circuits, and algorithm steps described in connection with theembodiments disclosed herein may be implemented as electronic hardware,computer software, or combinations of both. The various illustrativecomponents, blocks, modules, circuits, and steps have been describedgenerally in terms of their functionality. Whether the functionality isimplemented as hardware or software depends upon the particularapplication and design constraints imposed on the overall system.Skilled artisans recognize the interchangeability of hardware andsoftware under these circumstances, and how best to implement thedescribed functionality for each particular application. As examples,the various illustrative logical blocks, modules, circuits, andalgorithm steps described in connection with the embodiments disclosedherein may be implemented or performed with a digital signal processor(DSP), an application specific integrated circuit (ASIC), a fieldprogrammable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components such as,e.g., registers and FIFO, a processor executing a set of firmwareinstructions, any conventional programmable software module and aprocessor, or any combination thereof designed to perform the functionsdescribed herein. The processor may advantageously be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, programmable logic device, array of logicelements, or state machine. The software module could reside in RAMmemory, flash memory, ROM memory, EPROM memory, EEPROM memory,registers, hard disk, a removable disk, a CD-ROM, or any other form ofstorage medium known in the art. An exemplary processor isadvantageously coupled to the storage medium so as to read informationfrom, and write information to, the storage medium. In the alternative,the storage medium may be integral to the processor. The processor andthe storage medium may reside in an ASIC. The ASIC may reside in atelephone or other user terminal. In the alternative, the processor andthe storage medium may reside in a telephone or other user terminal. Theprocessor may be implemented as a combination of a DSP and amicroprocessor, or as two microprocessors in conjunction with a DSPcore, etc.

Preferred embodiments of the present invention have thus been shown anddescribed. It would be apparent to one of ordinary skill in the art,however, that numerous alterations may be made to the embodiments hereindisclosed without departing from the spirit or scope of the invention.Therefore, the present invention is not to be limited except inaccordance with the following claims.

1. A method of scheduling transmissions in a wireless communicationsystem, comprising: receiving a channel condition indicator sent by amobile station at a scheduler, determining a priority value for themobile station using a function:Priority(i)=Pilot_Power_Max−Pilot_Power(i), where Priority(i) is thepriority value for the ith mobile user, Pilot_Power_Max is the mobilestation's maximum pilot power, and Pilot_Power(i) is the mobilestation's pilot power at time of scheduling.
 2. The method of claim 1further comprising: calculating priority values for a plurality ofmobile stations as a function of the channel condition indicator; andselecting at least one of the plurality of mobile stations for asubsequent transmission based on the priority value.
 3. The method ofclaim 2 wherein: the channel condition indicator is based upon themobile station's transmit pilot power and requested data rate.
 4. Themethod of claim 2 wherein: The channel condition indicator is based uponthe mobile station's transmit power as determined by power controlcommands.
 5. A method of scheduling in a wireless communication system,comprising: receiving a channel condition indicator sent by a mobilestation at a scheduler, determining a priority value for the mobile userusing a function:Priority(i)=a(i)*(Pilot_Power_Average(i)/Pilot_Power(i)) wherePriority(i) is a priority value for an i-th mobile user,Pilot_Power_Average(i) is a mobile station's pilot power averaged over acertain period of time, Pilot_Power(i) is a mobile station's pilot powerat the moment of scheduling, and a(i) is a weighting factor.
 6. Themethod of claim 4 wherein: the weighing factor is based upon the mobileuser's speed.
 7. The method of claim 4 wherein: the weighing factor iscomputed according to a function:a(i)=(sector_throughput/user_throughput(i))̂b, where 0≦b≦1.
 8. Acomputer-readable medium including computer-executable instructions forscheduling transmissions, comprising: processing channel conditionindicators received from a plurality of mobile stations; calculating apriority value for each of a plurality of mobile stations; determining atransmission schedule for the plurality of mobile stations as a functionof the priority value.
 9. The program as in claim 7, wherein calculatinga priority value uses the function:Priority(i)=Pilot_Power_Max−Pilot_Power(i), where Priority(i) is thepriority value for the ith mobile station, Pilot_Power_Max is the mobilestation's maximum pilot power, and Pilot_Power(i) is the mobile user'spilot power at time of scheduling.
 10. The program as in claim 7,wherein calculating a priority value uses the function:Priority(i)=a(i)*(Pilot_Power_Average(i)/Pilot_Power(i)) wherePriority(i) is a priority value for an i-th mobile station,Pilot_Power_Average(i) is the mobile station's pilot power averaged overa certain period of time, Pilot_Power(i) is the mobile station's pilotpower at the moment of scheduling, and a(i) is the weighting factor. 11.The program as in claim 9, wherein the weighting factor is computedaccording to a function:a(i)=(sector_throughput/user_throughput(i))̂b, where 0≦b≦1.
 12. In awireless communication system, a network, comprising: receiving meansfor receiving channel condition indicators from a plurality of mobileusers; means for determining a priority value for each mobile station;means for determining a transmission schedule for a plurality of mobileusers, based on the priority value.
 13. The network of claim 12, whereinthe priority value is a function of:Priority(i)=Pilot_Power_Max−Pilot_Power(i), where Priority(i) is thepriority value for the ith mobile station, Pilot_Power_Max is the mobilestation's maximum pilot power, and Pilot_Power(i) is the mobilestation's pilot power at time of scheduling.
 14. The network of claim12, wherein the priority value is a function of:Priority(i)=a(i)*(Pilot_Power_Average(i)/Pilot_Power(i)) wherePriority(i) is a priority value for an i-th mobile station,Pilot_Power_Average(i) is the mobile station's pilot power averaged overa certain period of time, Pilot_Power(i) is the mobile station's pilotpower at the moment of scheduling, and a(i) is the weighting factor. 15.The network of claim 14, wherein the weighting factor is a function of:a(i)=(sector_throughput/user_throughput(i))̂b, where 0≦b≦1.
 16. Anapparatus in a wireless communication system, comprising: a processingelement; and a memory storage element coupled to the processing element,the memory storage element adapted for storing computer-readableinstructions for implementing: means for receiving a channel conditionindicator from a plurality of mobile stations; means for computing apriority value for each mobile station based on the channel conditionindicator; means for scheduling the plurality of mobile stations basedon the computed priority values.